1. Field of the Invention
The present invention relates to a circuit and method for symbol timing recovery in phase modulation systems, and particularly to a circuit and method for symbol timing recovery, which generates optimal sampling points only operating in a polar coordination.
2. Description of the Related Art
In the technology of a digital wireless baseband demodulation, π/4-DQPSK baseband demodulation has been widespreadly used. For example, USDC and PACS system in North America, and PDC and PHS system in Japan adopt π/4-DQPSK baseband modulation and demodulation to design their wireless system modems. The advantage of π/4-DQPSK baseband technology is high efficiency, high performance and easy to implement a receiver.
The prior π/4-DQPSK baseband modulation and demodulation technology modulates transmission signals at a transmission end, and convert the signals to phase representations   (            π      4        ,                  3        ⁢        π            4        ,                            5          ⁢          π                4            ⁢                           ⁢      and      ⁢                           ⁢                        7          ⁢          π                4              )which act as phase differences of continuous neighboring signals for representing transmission bit signals.
When demodulated at a receiving end, the received intermediate frequency (IF) signals are first converted into digital signals through an analog/digital converter, and then transmitted to a digital front end to generate a digital baseband in-phase signal In and quadrature signal Qn. Next, the signals are transferred to a rectangular coordinate to compute optimal sampling points, that are the same with points corresponding to the transmission signals at a transferring end. The above description is illustrated in U.S. Pat. No. 4,941,155, titled “METHOD AND CIRCUITRY FOR SYMBOL TIMING AND FREQUENCY OFFSET ESTIMATION IN TIME DIVISION MULTIPLE ACCESS RADIO SYSTEMS.” The prior art takes digital baseband in-phase signal In and quadrature signal Qn as input signals, and processes a symbol timing recovery method as follows:    (1) generating a phase difference Δθ of input signals;    (2) multiplying the phase difference by four;    (3) transferring the result from a polar coordinate to a rectangular coordinate;    (4) using 16 accumulators to compute 16 vector summations, wherein                                                         f              1                        ⁡                          (                              X                ,                Y                            )                                =                                                    (                                                      ∑                                          n                      =                                                                        16                          ⁢                          N                                                +                        1                                                                              ⁢                                      X                    n                                                  )                            2                        +                                          (                                                      ∑                                          n                      =                                                                        16                          ⁢                          N                                                +                        1                                                                              ⁢                                      Y                    n                                                  )                            2                                      ,                                      1          ≤          i          ≤          16                ;            and    (5) locating optimal sampling points at positions having the maximum value of fi (X,Y).
The computation process is very complex, and especially a lot of mathematical transformations are executed between the polar coordinate and rectangular coordinate. For the prior art, not only the computation method is trivial, but also the executing time is very long.